What's the purpose of metadata?

GIS practitioners generally blow off metadata. 'Yeah, yeah--metadata is important, I'll get around to writing it as soon as we finish updating the data itself at the end of the next leap-fiscal year.' And their lack of emphasis on it is not misplaced: their job is generally to make sure that their data is as accurate and current as possible. Someone who makes data generally knows where all of his data is.

But how does he find out where everyone else's data is? If Joe is in charge of the county land ownership file, why should he care where Susie's city parks file is? He only has to figure that out once every three years when he is banging out a basemap to justify his job or entertain his kid's Cub Scout pack.

For Joe and Susie, their daily need for metadata is to document data that is already found. So naturally, they don't think of it as serving a discovery function. The metadata sits right next to the data--it doesn't help anyone discover it. This is exactly like descriptive cataloging versus subject cataloging. The descriptive cataloger is just trying to accurately describe the item in hand. She doesn't care about making sure the item is findable. It's not her job.

So who does need a discovery mechanism? Where do description and discover meet? Who is searching for GIS data? Where do they start looking? What methods do they use? Do they act like other users with information needs?